The present invention applies to liquid tanks connected to liquid-solid separators such as centrifugal separators. While not limited to industrial and chemical engineering uses, the present invention is typically applied in these areas. The present invention is designed to overcome problems associated generally with operating liquid-solid separators connected to liquid tanks and more specifically with operating separators and tanks as components of a larger system.
For example, one application of the present invention is the method and apparatus of process control of liquid-solid separators connected to fluid tanks as part of the system for treating spent scrubbing medium from flue gas desulfurization wet scrubbers. In this process, sulfur dioxide-containing flue gas may be scrubbed with a slurry containing magnesium scrubbing components. The scrubbing takes place, for example, in an absorption tower in which the gas flow is typically countercurrent to and in intimate contact with a scrubbing solution. The solution may flow over packing or trays, or be sprayed into an open section of the tower.
Spent scrubbing fluid in a desulfurization wet scrubber is largely comprised of sulfites. This scrubbing fluid needs to be treated to convert sulfites into readily usable or disposable products such as magnesium hydroxide and calcium sulfate. To this end, the treatment process is started by removing a portion of the scrubbing fluid to an oxidizer. In the oxidizer the aqueous scrubbing fluid, comprised mainly of sulfites, is contacted at least with an oxygen-containing gas, such as air, which is typically sparged through the unit and removed from the top after contact with the fluid. The sulfites are converted to sulfates. This treated aqueous scrubbing fluid contains dissolved sulfates and suspended sulfates as well as other minor suspended solids. The fluid is then discharged to a first group, or bank, of liquid-solid separators.
The liquid-solid separator uses centrifugal force to separate the smaller low density particles from the larger high density particles. The oxidized medium is separated into an overflow of sulfates containing few suspended solids and an underflow with a heavier concentration of suspended solids. The overflow is clarified and passed to a regeneration tank.
In the regeneration tank, the oxidized and separated overflow is contacted with an aqueous slurry of magnesium-containing lime. The magnesium-containing lime slurry charged to the regeneration tank is formed in a slaker by the addition of magnesium-containing lime and water thereto. In the regeneration tank the sulfates are reacted with calcium hydroxide, which results in the precipitation of calcium sulfate and formation of magnesium hydroxide.
The regenerated aqueous medium is passed from the regeneration tank, by means of a pump, to a second bank of liquid-solid separators. The regenerated medium is separated into an aqueous overflow of mostly magnesium hydroxide and a underflow with a heavy concentration of calcium sulfate. The underflow is discharged into a dilution tank.
The underflow of calcium sulfate may be further contacted with additional solution in the dilution tank before the resulting aqueous solution is discharged to a third bank of liquid-solid separators wherein the solution is once again separated into an overflow and an underflow. The captured magnesium hydroxide in the overflow can be recycled back to the absorber tower and calcium sulfate in the underflow solution can be recycled back to the oxidizer.
Such a scrubbing process and scrubbing medium treatment system is described in U.S. Pat. Nos. 5,039,499 and 5,084,255, the contents of which are incorporated herein by reference and both patents are assigned to the assignee of the present invention.
In the above system, for example, the present invention comprises an on-line control method and apparatus for the controlling and coordination of the liquid tanks and liquid-solid separators. The coordination is necessary to help optimize the use of liquid-solid separators, optimize feed tank levels to avoid tank overflow, to maintain a continuous fluid flow and to help eliminate fluid line back pressure. One or more oxidizing tanks, regeneration tanks, and further dilution tanks may each be connected to a bank of liquid-solid separators.